Nick's problem with his depth sounder

[ash]

Hi All

Have you been keeping up to date with the adventures of Nick and Cathy? See Nick and Cathy's Website. Their depth sounder doesn't work properly since their relaunch.

"Only baffling technical problem at the moment is the echo sounder fluctuating wildly when the engine is at anythng above idle revs - any ideas?"

See HERE

Anyone got any ideas?

From memory - Fairwinds had a digital display - so assume it wasn't replaced and the problem has occured since the installation of the new engine.

Ash

[claymore]

Sounds like a suppressor problem and a good case being made for hanging on to old engines!

[ash]

Do you still need to retrofit suppressors? I remember fitting them to the dynamo/coil of my old cars to stop the buzz in the radio - but not for years.

a good case being made for hanging on to old engines!

There's old, and then there's really old - the regulator part of my combined regulator/starter solenoid for my dynastart started acting up again today!

Fairwinds has a retrofitted switchpanel - so the wiring is reasonably removed from the engine. I would have expected a new, modern alternator to have a clean output - though the output to the tacho must be pulsed.

I suppose that there is a possibility that Nick has broken the screen on the cable between the transponder and the head if he was fitting extra kit in the switchpanel. Is the screen just a screen or is it one of the conductors? Would it work at all?

Silkie - I seem to remember that you had the same interference problem from your outboard - did you resolve it?

Ash

[Silkie]

Silkie - I seem to remember that you had the same interference problem from your outboard - did you resolve it?

No, but told all I know here.

[ash]

No, but told all I know

Sorry - missed your reply.

I tend to use View Posts Since Last Visit but this can miss posts if we were both logged on at the same time on previous visit.

Ash

[Silkie]

I tend to use View Posts Since Last Visit but this can miss posts if we were both logged on at the same time on previous visit.

I bet you miss loads of good stuff like that!

[Nick]

The transducer cable is maybe to close to the tacho wire . . . perhaps if I can separate / shield it? What could I use to shield it, depleted uranium or lead?

Will have a look; thanks Ash for bringing this up.

[Telo]

Might not be an electrical problem. Is the transducer thingy too near the water turbulence caused by the propellor perhaps?

Our transducer is well forward of the prop, and works well in all circumstances except astern. When we go astern, it's fine for thie first few seconds when we experience the prop walk (to starboard in our case). However, once we get sternway, the signal disappears, and no amount of fiddling about with the gain control makes any difference.

My assumption is that the noise/turbulence/disturbance of the wash from the prop confuses the poor little thing.

Maybe Nick's transducer is just too near the prop to get a smooth flow.

Just a guess.

[Nick]

The transducer is forward of the keel at the cutaway forefoot, the prop is behind the rudder, so this seems unlikely. Common problem in astern because the water flow is under the boat, but you would have to have an aft-mounted transducer for it to be a problem in forward I think.

[cpedw]

I have no experience of it, but I believe transom mounting is used on some motorboats to avoid another hole in the hull. That could put the transducer right in the prop wash if you weren't careful. Presumably there's some advice from the manufacturers about that. Does anyone know more?

Derek.

[DaveS]

Nick,

Am I right in understanding that your sounder works fine under sail and slow motoring, and the problem only materialises when motoring at higher revs?

If so, I think we can probably discount water turbulence funnies etc.

To confirm this, can you replicate the problem while stationary by revving the engine in neutral? If so, it's definitely nothing to do with what's below the hull.

That brings us back to sources of electrical interference. While I wouldn't expect much noise off a properly functioning alternator, clipping on a few ferrites is quick to try, cheap, and can do no harm. The only worry is that you might be covering up something of deeper significance - like a loose connection, say, which might well cause electrical noise and, more worryingly, heat. If putting ferrites on the charging circuit wires cures the sounder problem I would want to then feel all the connections for temperature build up when the alternator is working hard: charging batteries after a long sail, say. (Stop the engine, obviously, before feeling any which are close to moving bits!) If everything's cool ( ) then it might be worth checking the alternator brushes: if they're near the end of their life they might be sparking.

The other likely cause is other electronics, particularly high powered active devices: other sounders / fish finders, radar, VHF / HF transmitters etc. A "smart" alternator regulator might also create problems, but ferrites should cure that easily. After that it gets a bit more esoteric, but I would work through the most likely causes first...

[Nick]

Nothing to do with water movement, only engine revs. Anything above tickover causes the problem.

What are ferrites, where do you get them and where do you put them?

If it is a loose connection, where is it likely to be, to the alternator or to the transducer?

[DaveS]

Nick,

Ferrite is sintered iron oxide - I've forgotten which oxide, but for our purpose it doesn't matter. It has high magnetic permeability, particularly at high frequencies. It is therefore a good material for the core of a radio frequency transformer or choke. A choke is basically a coil of wire which exhibits low impedence to DC - equal to the wire's resistance - and a higher impedence to AC, which rises with frequency.

Electrical noise is essentially high frequency, so if a choke is inserted into a noisy DC circuit the DC current is virtually unaffected, but the AC noise meets with a high impedence and is significantly reduced. With heavy wires it would be inconvenient to coil them up, so similar results can be obtained by passing them through hollow ferrite cylinders - a 1 turn choke in effect. This is a common technique for suppressing interference, and ferrites in all shapes and sizes are readily available. The most convenient are probably the split type enclosed in plastic which clip together over a cable without requiring it to be disconnected. Obviously you must choose a hole size bigger than the cable's diameter!

RS or Maplin would be the normal source here. They're cheap - a few pence each. I used some to successfully cure interference from my "smart" battery charger which was blocking Navtex reception.

If you've followed the argument above you'll realise that it doesn't matter much where on the noisy circuit you put them (and if putting on one doesn't solve the problem you can always add more), but it is conventional "wisdom" to place them near the notional source, i.e. probably near the alternator in this case.

If this cures the problem I do recommend then checking for a loose connection somewhere on the charging circuit: at the alternator positive output terminal, the negative output terminal (or earthing point if the negative is not isolated), at the batteries, switches, and any other connections. You can be posh and use a voltmeter to check for volt drops or, as I previously suggested, just feel everything: your fingertips should easily pick up the heating effect of a bad joint. One point I should have made before: doing this immediately after working the alternator hard rather than during will not only avoid getting yourself caught in the works, but will mean that disturbing a connection that falls apart when you touch it will not harm the (now stopped) alternator.

[Silkie]

No' bad Dave. That's a pretty reasonable paraphrase of my post on the stern gland thread.

PS - You should be careful showing off genuine knowledge like that on t'internet. People could get the idea (particularly now that you're retired) that you might be able to help them with, frinstance, their problems with charging a battery from an outboard motor.

[DaveS]

Oh, my fees are very reasonable...